One use for a contact lens is to change the color of one's eye. A contact lens that changes eye color has the design of the iris of the eye printed on the surface thereof. The ink that changes the eye color does not inhibit the user's visibility. The coloring in the highest quality contact lenses consists of numerous radially extending colored lines with the strands having various shades of color which combine to makeup the design of the iris. A suitable print pad must be used to apply thin lines of pigment containing inks to the smooth surfaces of a curved contact lens which minimizes the smearing the ink as it is applied or blending the ink into a previously applied coloring.
The ink that makes up the coloring may be applied directly to the outer surface of the lens after the lens has been manufactured, or may be printed into the concave inner surface of the mold used to manufacture the lens, such that when the lens is drawn from the mold, the pigment is already embedded into the outer surface thereof. Regardless of whether the ink is applied to a convex outer surface of a lens or a concave inner surface of a mold, the print pad that applied the ink must have qualities that allow the ink to be applied without smearing.
Existing print pads for applying ink to contact lenses and the like are made of a silicon rubber. The print pads have a somewhat blunted forward tip, and behind the forward tip the surface diverges in a complex curve until it reaches a generally cylindrical base. In the printing process, the print pad is first applied to a plate having etchings in the surface that are filled with ink. The ink is transferred to the surface of the print pad as the print pad is withdrawn from the plate. The print pad is next applied to the concave surface of a mold, or the convex surface of a contact lens. It is the very tip of the print pad that makes first contact with either the etched plate or the surface to which the ink is to be applied. Following the initial contact, the tip deforms as the pad is applied against the surface such that successive portions of the print pad make contact with the plate or print surface in a widening annular pattern.
In order for the print pad to apply the ink evenly to a print surface without causing smearing, the material of which the print pad is made must have a desired durometer. Even so, it has been found that the ink applied near the center of a contact lens is often smeared whereas the ink that is applied near the outer perimeter thereof is not. This is because the tip of the print pad undergoes considerably greater compression than does the more rearward portions of the print pad. The uneven forces within a print pad as it deforms limits the quality of the printed reproduction of the human iris on a contact lens. There is therefore a need to improve the quality of the print pads that apply ink to the surface of a lens or the surface of a mold used to manufacture a lens.